1. Field of the Invention
The present invention relates to a television receiver comprising a signal processing system that separates a composite video signal into a luminance signal and a chrominance signal and performs high picture quality processing such as scanning line interpolation on the two signals.
2. Description of the Prior Art
The improved television receiver known as EDTV (extended definition television) generally works as follows. The receiver first separates a composite video signal into a luminance signal and a chrominance signal, converts the two signals to digital format, submits the digitized signals to scanning line interpolation for doubling the number of the scanning lines involved, and subjects the resulting signals to half-time compression. After their conversion to digital format followed by the scanning line interpolation and time compression, the luminance signal and the chrominance signal are converted back to analog format and are supplied to a matrix circuit. The matrix circuit generates a red, a green and a blue signal whose rate is double the normal rate. The double rate red, green and blue signals, with their scanning line counts being twice as many as normal, provide non-interlaced scanning for high picture quality.
The scanning line interpolation method under the above scheme involves the use of a circuit that determines whether a supplied video signal is a motion picture signal or a still picture signal. If the supplied video signal is found to be a motion picture signal, an interpolation signal is generated using data in a field. If the detected signal is a still picture signal or something intermediate between motion and still picture, an interpolation signal is generated using data in other fields such as the one preceding the current field or subsequent thereto (the data is called out-of-field data).
How the above-described scanning line interpolation method basically works is described with reference to the model in FIG. 11. In the model of FIG. 11, the ordinate axis represents the vertical direction. Some of the 262.5 scanning lines are shown using circular cross sections thereof. The abscissa represents time base, on which the n-th field through the (n+3)th field of a video signal are indicated.
As evident from FIG. 11, the interlaced scanning as per the NTSC system is realized because the scanning line positions are shifted in the vertical direction between odd-numbered and even-numbered fields of a video signal. The scanning line interpolation described above involves interpolating the scanning lines in each field. For example, broken line circles (.alpha., .beta., .gamma., etc.) are interpolated in the (n+2)th field. (In FIG. 11, the interpolated scanning lines in the other fields are not shown.)
Consider the interpolation of scanning lines for a signal .alpha.. First, the signal a in the (n+3)th field is compared with the signal d in the (n+1)th field, the latter field being delayed by 525H (i.e., by one frame). A check is made at this point to see if the picture involves motion. If the picture is found to be moving, it is not appropriate to generate the signal .alpha. by mixing the signals a and d of other fields. Instead, the signal .alpha. is generated by mixing the signals b and c which represent the scanning lines above and below the signal .alpha.. That is, the interpolation in this case is carried out using only the data in the (n+2)th field.
If the comparison of the signal a with the signal d shows the picture to be still, the signal .alpha. is generated by mixing the signal a in the (n+3)th field with the signal d in the (n+1)th field, each of the latter two signals being shifted by a time period of 262H in the direction of the signal .alpha.. That is, the interpolation in this case is performed using the data in fields other than the (n+2)th field.
The number of scanning lines per field is usually 262.5. With signals of this standard scanning line count, there is no problem as long as they are interpolated using inside-field data when found to be motion picture signals, or using out-of-field data when found to be still picture signals. In some cases, however, there may be supplied video signals whose number of scanning lines per field is not 262.5 (262 to 263).
One such case involves a video disc player that is brought to a stop while supplying a video signal. The stopped state causes the video disc player to supply non-standard signals, their scanning line count per field alternating between 261.5 and 263.5. In the stopped state, a blue screen typically appears on the display with no image contained therein. However, if characters are displayed, they dislodge vertically. For example, a character "A" in FIG. 12 (A) appears dislocated as shown in FIG. 12 (B).
The reasons for this phenomenon are explained with reference to FIG. 13. Where the signal .alpha. is to be interpolated, the signal a is first compared for motion detection with the signal d which is delayed by 525H. (The signal d applies here since 261.5+263.5=525.) In this case, the signal is found to be a still picture signal. Thus the signal .alpha. is generated by shifting the signals a and d by 262H in the direction of the signal .alpha. and by mixing these two signals. However, if the number of scanning lines per field alternates between 261.5 and 263.5, the position of the signals a and d after the temporal shift of 262H actually corresponds to the position of a signal .alpha. s. Likewise, the interpolation using signals c and e results not in a signal .gamma. but in a signal .gamma. s. It is for these reasons that the picture dislodges vertically when a non-standard signal is supplied.